Since moving to England from Leinster in 2005, Cullen - 28 - has made 45 appearances for the Tigers, including 10 as captain.
Cullen is the second Tiger to re-join Leinster following from the recent signing of Shane Jennings and is the fourth to commit to the province in recent weeks following that of captain Brian O'Driscoll and Felipe Contepomi.
Leo Cullen commented: "Leaving Leicester was a difficult decision, and I had to think long and hard about it. I have really enjoyed my time with the Tigers and I feel that I'm a better player for the experience. The training environment is second to none and it has been great to have been part of such a prestigious club.
"We are still well placed in all three competitions and it would cap off two great seasons personally if I could help play a part in winning some much sought after silverware.
"I am sorry to be saying goodbye to Leicester Tigers but when I got the opportunity to come home to Leinster, I knew the time had come to do so. I am a Leinsterman born and bred, my family and friends are all here, and this is where I want to be."